Conventionally, the measurement of cardiac output by right heart catheterization, which is performed in a cardiac function inspection, employs the indicator dilution method. This indicator dilution method is classified into three categories: the thermal dilution method in which the cardiac output is obtained by utilizing thermal diffusion, the dye dilution method in which the cardiac output is obtained utilizing variations in the illuminance which are caused by the diffusion of a dye, and the electrolyte dilution method in which the cardiac output is obtained utilizing variations in the resistance caused by the diffusion of an electrolyte. The thermal dilution method will be described below in detail.
In right heart catheterization, a catheter 4 is introduced into the body from the cervical vein, the femoral vein, the basilic vein or the like. The forward end of the catheter passes through the superior vena cava or the inferior vena cava, the right atrium and then the right ventricle, and finally reaches the pulmonary artery, as shown in FIG. 16. The catheter 25 has a injection port 26 and a thermistor 1, which are respectively positioned in the right atrium and in the pulmonary artery when the forward end of the catheter 25 is located within the pulmonary artery. When a liquid having a temperature higher or lower than that of the blood is ejected into the right atrium from the injection port 26, it is diffused in the right atrium and the right ventricle and is diluted by the blood. The temperature of the liquid diluted is detected by the thermistor 27 located within the pulmonary artery to obtain a dilution curve of the temperature detected (which is a graph of the variations in the temperature with time), and the cardiac output is calculated from the area or the like of that dilution curve by the Stewart Hamilton Method using the following equation (1). ##EQU1## where CO is the cardiac output, Si is the specific gravity of the injectate, Ci is the specific heat of the injectate, Vi is the volume of injectate, Ti is the temperature of the injectate, Tb is the temperature of the blood, Sb is the specific gravity of the blood, Cb is the specific heat of the blood, and ##EQU2## is the area of the thermal dilution curve. Calculation of the cardiac output by the Stewart Hamilton Method is known as being relatively accurate. Cardiac output measuring apparatuses which perform measurement on the basis of this method have been available.
However, such a conventional cardiac output measuring apparatus requires a relatively high degree of skill and troublesome operation. Hence, there has been a demand for an apparatus which is simple and easy to use and which ensures accurate measurement. The inventors of this application have studied the measurement of the cardiac output by the dilution method, in particular, the measurement of the cardiac output by the thermal dilution method, and have found the following five problems with the conventional measuring apparatuses.
(i): As can be seen from the above equation, in the thermal dilution method, the volume of the injectate and the temperature thereof greatly affect the accuracy of the calculation. Normally, after the temperature Ti of the liquid has been accurately measured by a thermistor or the like, the catheter 26 is primed and the indicator is then injected into the blood vessel. However, the indicator may be cooled or heated by the body temperature before it enters the blood vessel, causing measurement errors. More specifically, when the indicator having a temperature set to the correct value is injected, the temperature of the liquid is raised or lowered by the body temperature from its initial value at the beginning of the injection, the liquid having substantially its original temperature being injected a certain period of time after the injection has started. This can be the cause of measurement errors, because the total heat capacity of the injectate into the blood vessel is an unknown value. The heat capacity is mainly determined by the capacity of the catheter, the volume Vi of the liquid to be injected and the injectate temperature Ti. The capacity of the catheter is a factor ignored in the Stewart Hamilton Method. Generally, the capacity of the catheter is determined in accordance with the outer diameter (French size) of the catheter. Conventionally, the heat capacity is estimated using the volume of the injectate, the catheter size and so on, and a correction constant is obtained by comparing the experimentally obtained cardiac outputs with these estimated values. A more accurate cardiac output, which is not affected by the residual liquid, is obtained by utilizing this correction constant.
However, the aforementioned correction constant which is conventionally obtained using the volume of the liquid remaining in the catheter is a numeric value which is meaningless to the operator, and this renders it liable to be erroneously calculated or erroneously input to the measuring apparatus.
(ii): In the above-described measurement of the cardiac output on the basis of the thermal dilution method which is expressed by Equation (1), it is essential to accurately measure the temperature Ti of the indicator to be injected, because the temperature Ti not only appears in the term (Tb-Ti) in the above-described equation and but also has a relation with the heat capacity of the residual liquid which can be the cause of measurement errors, as described in (i).
Generally, the indicator is immersed in an ice-cooled or warmed solution whose temperature is kept constant. The temperature of the ice cooled or warmed solution in which the indicator is immersed may therefore be regarded as the temperature T1 of the indicator in a case where a skillful operator handles it or in a case where the desired cardiac output value is not an absolutely precise one. However, conventionally, measurement of the injectate temperature is performed for each measurement. This makes the cardiac output measurement inefficient.
In other words, in the cardiac output measurement apparatus which employs the dilution method, it is anticipated that, when it is determined that the operator is not going to perform measurement of data or when the apparatus detects an instruction of non-measurement of data, the manually preset data is used.
(iii): As can be seen in the graph in FIG. 17, sampling of the blood data required for the calculation of the area (integration) of the dilution curve has to be started within a certain period of time after the injection of the indicator. Starting of the calculation of the integration a long time after the blood temperature has dropped may cause calculation errors. However, conventionally, the indicator injection side may be apart from the measuring apparatus installation side. In that case, one of the two operators who is in charge of the injection of the liquid gives an instruction to the other after the liquid has been injected, and the other operator starts measurement, i.e., starts integration. In consequence, there has been always the possibility of measurement errors. In a case where the single operator takes care of both operations, he or she must do them quickly in a short period of time.
Not only the thermal dilution method but also the other indicator dilution methods involve these measurement errors and the troublesome operation.
(iv): In the above-described cardiac output measurement method which employs the thermal dilution method or the other indicator dilution methods, measurement of the cardiac output is performed intermittently each time the injector is injected. It is therefore impossible to continuously measure the cardiac output. If measurement is performed frequently, the total volume of the injectate increases, increasing the burden on the examinee and increasing the risk that the examinee may be infected during the liquid injection.
In order to eliminate the disadvantages experienced by the conventional cardiac output measurement which is based on the indicator dilution method, in particular, the thermal dilution method and which is only capable of intermittent measurement, the present inventors have proposed under Japanese Application No. 59-244586 (in the specification of Japanese Patent Laid-Open No. 61-125329) an improved cardiac output measuring apparatus which is capable of continuously measuring the cardiac output. This continuously measuring apparatus involves the measurement of the cardiac output over a long period of time and continuous recording of the cardiac output.
(v): Subject matter of Japanese Application No. 59-244586 is to obtain the relation between the cardiac output and the blood flow velocity which are actually obtained in the measurement based on the thermal dilution method and to operate the cardiac output thereafter from the thus-obtained relation and the blood flow velocity actually measured. Because the cardiac output and the blood flow velocity have a certain relation, if the blood flow velocity obtained is not an absolute one, relative changes in the cardiac output can be obtained using that relation.